Today's complex semiconductor chips can have ten or more levels of metallization. Since some degree of surface non-planarity is typically introduced at each level, the surface non-planarity, in general, will become greater as more metal levels are fabricated. A three dimensional surface height map of one chip is shown in FIG. 1. The map shown in FIG. 1 was generated using data obtained from a lithography tool prior to exposure of an upper metal level. The peak of FIG. 1 represents a surface irregularity, which represents a bad focus area. It should be recognized though, that other peaks of FIG. 1 may represent irregularities in the surface, but these peaks do not necessarily represent a bad area of focus as this would depend on the focal planes used by the lithography tool.
Lithography tools used today expose wafers by scanning a slit (essentially a long rectangular opening through which light passes through the reticle, through lens elements, and onto the wafer) across the reticle field. Using optical or mechanical sensors, the lithography tool continuously reads the position of the wafer surface at multiple points within the slit as it scans, or reads the entire wafer surface prior to scanning. The tool must choose and expose with a best average focal plane across the slit.
Previous generation tools, e.g., single stage lithography tools, measure the surface topography in real time, during the exposure scan, but newer, multiple stage tools can pre-measure the entire wafer surface on the “idle” stage prior to the exposure scan, for increased throughput. The plane of exposure can be moved up and down, and rotated around two axes in order to achieve the best average focal plane at any particular instant, which is continually adjusted as the slit scans.
FIGS. 2 and 3 show two dimensional surface profiles, taken by measuring the surface height along a surface. FIG. 2, in specific, is a two dimensional surface profile taken by measuring the surface height in a direction over the large surface “peak” in FIG. 1. (It should be recognized that the profile is not meant to be an accurate representation of the surface height data and is provided for illustrative purposes only). FIG. 2 also shows a two dimensional representation of the plane, seen on edge, that the lithography tool might choose as the best average focal plane, if it had to consider only this single two dimensional profile. As should be understood by those of skill in the art, the exposure plane shown by the line in FIG. 2 can be tilted (in a direction towards and away from the surface of the paper) in order to provide a better focus for some points on the surface of the wafer.
Some areas of the photoresist film which cover the wafer surface at the time of exposure, will inevitably be in better focus than other areas. For example, point A is obviously furthest from the best average focal plane (e.g., the distance along the axis of illumination from the best average focal plane), and point A will therefore have the worst average focus, for points along this particular profile. Point B, on the other hand, should have much better average focus than point A. Since chip designs can vary widely, an infinite variety of surface profiles are possible.
The surface topography can be modeled by empirical commercial chemical mechanical polishing (CMP) modeling programs which take into account the details of the metal pattern at a particular level and also the underlying topography from prior levels. In one example, the design data is fed into the modeling program (after model setup/calibration is performed), and the model output is a surface height above a reference point, anywhere in the chip design. In this example, the surface height is the weighted average of the average copper height and the average dielectric height about a certain reference point. Typically the copper thickness, dielectric thickness, and surface height are mapped in terms of square regions of a specific size (tiles); although, it is understood that the results can be mapped and viewed in various ways.
However, current modeling cannot accurately predict all of the areas that will be problematic for lithography. For example, current methodologies involve simply looking for high or low points within the CMP modeling surface height data, without taking into consideration the way in which the lithography tool decides the focal planes to use as it scans. More specifically, using the empirical modeling data and referring back to FIG. 3, the points indicated by the arrows labeled “C” and “D” which have the same height above some reference plane, might be considered to be “high” points along the curve (along the surface profile), at risk of bad focus (they are higher than most points along the curve). But because of the way that the lithography tool must select the best average focal plane (simulated on edge by the line labeled “Best Average Focal Plane”), the point indicated by the arrow labeled “C” will be exposed with much better focus than the point indicated by the arrow labeled “D”. Therefore surface height alone is not necessarily an accurate indicator of whether focus will be good or poor. Also, the point indicated by the arrow labeled “E” is a peak with respect to its immediate surroundings, yet it will be exposed with good focus. Therefore identification of such local “peaks” or “dips” is not necessarily an accurate indicator of whether focus will be good or poor.
Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.